In Study Item of LTE-Advanced (Long Term Evolution Advanced) of 3GPP (3rd Generation Partnership Project), the introduction of radio relay stations (hereinafter “RNs: Relay Nodes”) has been examined. The RN is one of techniques for increasing the communication speed of a mobile station (hereinafter “UE: User Equipment”) located at a cell edge and for expanding cell range of a base station (hereinafter “eNB: Evolved Node B”). Note that “cell” means the coverage area of a base station.
In 3GPP, a base station (eNB: Evolved Node B) having a function of connecting with an RN is called “Donor eNB (hereinafter DeNB)”. In this specification, the term “DeNB” is used to distinguish a DeNB from an ordinary eNB only when matters relating to connection with RNs and thus unique to DeNBs are explained.
Further, in this specification, a mobile station (hereinafter “UE: User Equipment”) that belongs to a DeNB without any RN therebetween is called “eNB-UE”. In contrast to this, a mobile station that belongs to an RN is called “RN-UE”. Further, when matters common to both eNB-UEs and RN-UEs are explained, they are simply referred to as “UEs”.
Further, in this specification, radio interfaces that form connections between a DeNB and an RN and between an upper RN and a lower RN are called “backhaul links”. Meanwhile, radio interfaces between an eNB and an eNB-UE and between an RN and an RN-UE are called “access links”. The backhaul link and the access link may use the same radio frequency (In-band mode), or may use mutually different radio frequencies (Out-band mode).
RNs can be classified according to their relay operation (according to their layer). In the simplest relay operation, amplifying and re-transmitting a radio signal transmitted from a DeNB are only performed. An RN like this, which performs a relay operation that is completed within a PHY layer, is called “Layer 1 Repeater”. Layer 1 Repeater does not perform decoding and re-encoding for data transmitted from a DeNB. In contrast to this, an RN that performs decoding and re-encoding for data transmitted from a DeNB is called “Layer 2 Relay”. Note that in Layer 2 Relay, scheduling of radio resources are dependent on the DeNB. An RN that has extended functions of Layer 2 Relay and thus has substantially the same functions as those of an eNB is called “Layer 3 Relay”, “Layer 3 Wireless Router”, “Self-backhauled eNB”, or the like. Layer 3 Relay has its own cell ID and performs its own scheduling, mobility management, and so on. This specification mainly discusses Layer 2 Relays and Layer 3 Relays.
Further, in 3GPP Release 9, MBSFN (Multimedia Broadcast multicast service Single Frequency Network) is standardized. MBSFN provides MBMS (Multimedia Broadcast multicast service), which is a broadcast type service, in a SFN (Single Frequency Network). In MBSFN, plural neighboring base stations use the same frequency resource and simultaneously transmit the same multicast/broadcast data (MBMS data). In the following explanation, an MBMS-data-encoded down link signal which is synchronously transmitted from plural base stations is called “MBSFN signal”.
UEs cannot distinguish an MBSFN signal simultaneously transmitted on plural cells from an MBMS signal transmitted on a signal cell. That is, an UE which located at cell boundary observes an MBSFN signal that arrives from plural cells as a multipath signal. In LTE in which an ODFM is used as the downlink communication scheme, it is necessary to design a system in such a manner that the multipath delay of an MBSFN signal remains within the guard interval of an ODFM signal in order to prevent the deterioration of the reception characteristic of the MBSFN signal. To reduce the multipath delay, plural base stations belonging to a MBSFN synchronization area transmit MBSFN signals, into which the same MBMS data is encoded, on the same time based on MBMS scheduling information.
FIG. 1 shows a logical architecture of an MBSFN. Evolved Node Bs (eNBs) 901A to 901C form cells 902A to 902C respectively, and provide an MBSFN service to an UE 903. In this specification, the cells 902A to 902C that provide an MBSFN service are called “MBSFN service cells”. Meanwhile, a cell that provides an ordinary unicast service to an UE is called “unicast cell”. Note that the MBMS service is provided by using the same subcarrier as that of the unicast service in a time-division manner. That is, the MBSFN service cells 902A to 902C do not provide the MBMS service at all times, but also provide a unicast service to the UE 903.
An MME 904, an MCE 905, and an MBMS GW 906 perform control of providing a MBMS service by MBSFN. The MME (Mobility Management Entity) 904 is connected with the eNBs 901A to 901C by means of an S1-MME interface, and performs mobility management and session management of the UEs 903 belonging to the cells 902A to 902C.
The MCE (Multi-cell/multicast Coordination Entity) 905 is an entity included in E-UTRAN, and is connected with the eNBs 901A to 901C by means of an M2 interface. The M2 interface is a control plane (C-Plane) interface relating to providing an MBMS service. The MCE 905 determines a radio resource (time and frequency resource), a modulation scheme, an encoding scheme, and the like that are used for an MBSFN operation of the eNBs 901A to 901C within an MBSFN synchronization area, by using the M2 interface. The MCE 905 supplies “MBMS scheduling information”, which indicates the radio resource (time and frequency resource) and the like used for the MBSFN operation, to the eNBs 901A to 901C.
Further, the MCE 905 is connected to the MME 904 by means of an M3 interface. The M3 interface is a C-Plane interface between an E-UTRAN and an EPC (Evolved Packet Core). The MCE 905 starts or stops an MBMS session in response to a message indicating the start or stop of the MBMS session, issued from an EPC (specifically, from MME 904).
The MBMS GW 905 is connected with the eNBs 901A to 901C through an M1 interface. The M1 interface is a user plane (U-Plane) interface relating to providing an MBMS service. The MBMS GW 905 transmits MBMS data (MBMS packet) to the eNBs 901A to 901C by using IP multicast.
The switching between the MBSFN service and the unicast service in the MBSFN service cells 902A to 902C is carried out on a subframe basis. According to the regulations of LTE that are compatible with LTE-Advanced, the downlink transmission and the uplink transmission are carried out on a 10 ms radio frame basis. FIG. 2 shows a structural diagram of a radio frame in accordance with LTE FDD (Frequency Division Duplex). One radio frame is composed of ten subframes (#0 to #9). Each subframe is composed of two slots. Each slot is 0.5 ms in length. Each slot includes a plurality of (NSYMB pieces of) OFDM symbols in the time domain. A radio resource defined by one OFDM symbol in the time domain and one subcarrier in the frequency domain is called “resource element”. The resource element is the minimum allocation unit of radio resources in the LTE/E-UTRAN downlink that adopts the ODFM (Orthogonal Frequency Division Multiplexing). Further, a resource unit defined by consecutive NSYMB OFDM symbols (equivalent to one slot) in the time domain and consecutive NSC OFDM subcarriers in the frequency domain is called “resource block”. In the case of LTE downlink with ordinarily-used cyclic prefix, except for a case where special multipath environments need to be supported, the value of NSYMB is seven and the value of NSC is twelve.
The switching between the MBSFN service and the unicast service in the MBSFN service cells 902A to 902C is explained with reference to FIG. 3. FIG. 3 shows a specific example of scheduling of MBSFN subframes and ordinary subframes. In FIG. 3, a cell 912 is a unicast cell that does not provide the MBMS service. The MBSFN service cells 902A to 902C transmit MBMS data in predetermined subframes according to “MBMS scheduling information” supplied form the MCE 905.
For the transmission of MBMS data that is multicast information to be received by plural UEs, a MCH (Multicast channel) and a PMCH (Physical Multicast Channel) are used as the transport channel and the physical channel respectively. A subframe onto which a PMCH is mapped is called “MBSFN subframe”. To make it possible to discriminate between an ordinary subframe onto which a PDSCH (Physical Downlink Shared Channel) used to transmit unicast data is mapped and a MBSFN subframe, different RSs (Reference signals) are assigned to different resource elements in these two types of subframes.